Traditional TV transmissions have been analog by nature. At the priority date of this patent application, the advent of widespread digital TV networks is seen. At least for a certain transition period there will exist a need for hybrid receivers that are able to receive both analog TV transmission of the conventional kind and digital TV and multimedia transmissions. The potential for two-way transmissions in digital multimedia networks suggests that the word “terminal” should gradually replace the word “receiver”. In this patent application the word “terminal” is broadly used to designate the device or a number of mutually interacting devices which a end user uses to exploit the services offered through a network or a number of networks which together arc capable of conveying both analog television transmissions and digital multimedia traffic.
The reception and processing of analog signals on one hand and digital signals on the other are tasks so different from each other that large portions of the terminal need to be duplicated in order to make it capable of hybrid operation. Combining analog and digital operation has proven to be somewhat problematic regarding some functions of the terminal, like the generation of on-screen display functions, also known as OSD functions for short. OSD functions are typically used as a graphical aid to the user of a terminal during e.g. local configuration and/or control operations. FIG. 1 illustrates certain parts of a prior art hybrid terminal. A digital transport stream is fed into a digital video decoding unit 101 where a demultiplexer 102 separates a video component from it. The separated video component goes into a video decoder 103 which here is an MPEG decoder (Motion Picture Experts Group); the decoder must naturally be selected according to the encoding scheme used in composing the digital video signal. The result of decoding is taken from the video decoder 103 to a graphics generation circuit 104 which also receives OSD-related commands from a processor 105, which in turn has received the OSD-related information e.g. from the user through a remote controller and an infrared link. The task of the graphics generation circuit 104 is to generate the picture frames, selected ones of which may have a part of the frame overlapped by an OSD object such as a menu or an information table.
The output of the graphics generation circuit 104 is coupled in an encoder unit 106 that converts the frames generated in the graphics generation circuit 104 into a suitable standard format accepted by a television set. The PAL, NTSC and SECAM formats are shown as examples in FIG. 1. The encoder unit 106 also generates a fast blanking-type switching signal.
A received analog video signal is coupled to a converter 111 that simply converts it from one analog video signal format to another analog video signal format; such converting is naturally unnecessary if the analog video signal is already in a format accepted by a television set. The standard format and fast blanking outputs of the digital video decoding unit 101 as well as the output of the converter 111 are all coupled to an RGB switch 121 that selects, as controlled by the fast blanking signal, the RGB source to be conveyed through to the television set. The RGB switch is also known as the switching matrix.
Adding OSD to a received analog video signal would most naturally be accomplished by using the same OSD processor and the same graphics generator that are used to add OSD to the digital video signal. However, attempting such a simple solution is known to cause problems. These are mainly related to the difficulty of synchronising the local generation of OSD to the received analog video signal in order to make the graphical OSD objects appear at correct locations on the display. For example a frequency modulated analog video signal received through a satellite includes a certain quantity of impulse noise with relatively large amplitude. If a simple and inexpensive sync signal separator like the known LM1881 of National Semiconductor Inc. is used to extract the horizontal and vertical synchronisation pulses from the received analog video signal, such impulse noise causes interference peaks at the sync separator output which in turn may cause the straight lines of the OSD objects to distort.
A straightforward solution would be to employ a more elaborate sync signal separator. Advanced circuits exist that include inherent noise filtering and pulse regenerating capabilities. However, in their off-the-shelf form such circuits are typically relatively old and expensive. Additionally they usually require a large number of auxiliary components and they may even necessitate some individual tuning, which makes them unattractive to designers of home appliances.
Another known way to solve the problem is to separate the sync signal from the received analog video transmission as usual but to use it directly to synchronise the digital video decoder. If it proves necessary to filter the sync signal this could be performed by appropriate software in the video decoder itself. The drawback of this solution is that it reserves a relatively large amount of processing capacity from the video decoder's processor. Additionally it is usually not advantageous to run the video decoder in slave mode, where its correct operation depends on the continuous reception of adequately clear external sync signals. Noise in the received analog video signal may result in distorted or interrupted operation.
If one is allowed to do major re-engineering on the video decoder, it is possible to integrate filtering and synchronisation hardware therein that could solve the aforementioned problems. However, at the priority date of the present patent application it would be more advantageous if a more conventional video decoder could be used.